1. Field of the Invention
The present invention relates to a camera, a lens apparatus, and a camera system for driving control of a lens.
2. Description of the Related Art
Conventionally, cameras or image-taking lenses which have an autofocus function have been frequently used.
The autofocus function is realized by detecting the focusing state of an image-taking lens to find a defocus amount, calculating a movement amount of a focus lens corresponding to the defocus amount, and driving the focus lens by the movement amount derived from the calculation. Driving control of the focus lens by a motor is absolutely necessary for the autofocus function.
A typical method of the lens driving control involves accelerating a motor or a lens to a target speed (acceleration control), driving the motor or lens at a constant sped equal to the target speed (constant-speed control), then decelerating the motor or lens for stopping it precisely at a target position (deceleration control), and finally stopping the motor or lens. All the accelerating control, constant-speed control, and decelerating control are speed control in which the rotational speed of the motor or the moving speed of the lens is detected at a sampling rate, the detected speed is compared with a target speed, and the motor is decelerated if the detected speed is higher than the target speed or the motor is accelerated if the detected speed is lower than the target speed, thereby maintaining the target speed or reaching the target speed.
When a DC motor, for example, is used as the motor, acceleration or deceleration is performed by increasing or reducing the driving effective voltage or the driving effective current. Alternatively, when a vibration type motor is used in which periodic wave voltages (alternating voltages or pulse voltages, etc.) with two different phases are applied to piezoelectric elements to excite vibration of a vibrator such that a contact body is relatively moved, acceleration or deceleration is performed by changing the frequencies, voltage values, or a phase difference of the aforementioned periodic wave voltages.
The lens driving control is preferably achieved as fast as possible for smooth autofocus operation. The control of a focus lens, however, requires high accuracy of stop position of the lens at a target position to maintain focusing accuracy. To hold the high accuracy of stop position, the focus lens needs to be decelerated sufficiently to a low speed and then stopped.
To address the need, various approaches have been adopted. For example, Japanese Patent Application Laid-Open No. H5(1993)-333257 has proposed a method of maintaining stable accuracy of stop position and driving time without being affected by load variations associated with the direction of a lens apparatus or ambient temperature.
The method of lens driving control employs a means for detecting the direction of the lens apparatus and ambient temperature. When the detection result shows that a heavy load is applied, the load of a lens itself can serve as a brake to perform sharp deceleration, so that a larger deceleration area (an amount of lens driving from a deceleration start position to a target position) is set. On the other hand, when the detection result shows that a light load is applied, a smaller deceleration area is set since the load of the lens itself does not serve as a brake to make the deceleration difficult.
Since the focal depth varies depending on setting of a stop (diaphragm, etc.), the required accuracy of stop position of a focus lens can be relaxed when the aperture of the stop is narrowed. With attention focused on this fact, Japanese Patent Application Laid-Open No. H7(1995)-77648 has proposed a method of controlling the stop position of a focus lens in combination with detection of the state of a stop.
In a lens with a variable focal length such as a zoom lens, the focal depth is larger on the wide-angle side, while the focal depth is smaller on the telephoto side. Thus, a focus lens is moved by a larger amount on the wide-angle side and by a smaller amount on the telephoto side to correct the same defocus amount.
High accuracy is necessary for the focus lens stop position on the telephoto side, but relatively low accuracy is allowed on the wide-angle side as compared with the telephoto side.
Conventionally, however, final accuracy of stop position of the focus lens is determined on the basis of the high stop accuracy required on the telephoto side. As a result, the accuracy of stop position of the focus lens is unnecessarily high on the wide-angle side to need a longer time period to correct the defocus amount.
The method of lens driving control proposed in Japanese Patent Application Laid-Open No. H5(1993)-333257 aims to minimize the driving time in consideration of load variations associated with the direction of a lens apparatus and ambient temperature. The method, however, does not lead to solution of the aforementioned problem. In addition, the method of lens driving control proposed in Japanese Patent Application Laid-Open No. H7(1995)-77648 aims at power savings, and does not result in solution of the aforementioned problem.